The invention relates to a method for operating an internal combustion engine wherein a fresh air charge of a combustion chamber is considered in the determination of a pressure in a region lying upstream from an inlet valve or, for the determination of the fresh air charge of a combustion chamber, the pressure in the region is considered lying upstream from the inlet valve. An rpm of a crankshaft of the engine is also considered in the determination.
Such a method is known from the marketplace and is used, for example, in internal combustion engines having intake manifold injection. In internal combustion engines of this kind, either an air mass sensor is installed in the vicinity of a throttle flap or an intake pressure sensor is installed in an intake manifold. For the control of the engine, one needs as a rule, however, the intake manifold pressure as well as the fresh air charge. This means that the quantity, which in each case is not detected with a sensor, must be simulated by means of a model. The corresponding model is characterized as a xe2x80x9ccharge exchange modelxe2x80x9d.
Based on this charge exchange model, the fresh air mass, which is inducted by the engine, is computed, for example, from the input quantity xe2x80x9cintake manifold pressurexe2x80x9d. The computation takes place by means of a linear equation which includes a linear slope factor which is multiplied by the difference between the intake manifold pressure and a partial pressure of an internal residual gas.
By considering this internal residual gas, the fact is taken into account that the cylinder charge always contains a certain residual gas quantity from the last combustion. A certain portion of the exhaust gas from the exhaust-gas pipe again reaches the combustion chamber during an exhaust gas recirculation because of valve overlap. This can, for example, be achieved in that the outlet valve closes only after the piston of the engine passes the top dead center. In this way, a time span can result wherein the outlet valve and the inlet valve of a combustion chamber are opened simultaneously. This time span is characterized as an overlap angle referred to a camshaft revolution.
From the marketplace, functions are known for computing the internal partial pressure of the residual gas in the combustion chamber as well as for computing the linear slope factor with the aid of characteristic fields. The following are, for example, fed into the characteristic fields: the rpm of the crankshaft of the engine; the overlap angle of the camshafts and, if required, the overlap centroid of the camshafts. However, such characteristic fields require a relatively large memory space. Furthermore, there is a requirement in present day internal combustion engines that the fresh air charge and/or the intake manifold pressure be computed with still greater precision.
From the marketplace, simulation programs are known with which the thermal and dynamic conditions within the engine can be simulated in very small steps. The actual operations during charge exchange can be simulated rather well with such simulation programs. Even pulsations which occur during operation in the intake manifold and in the exhaust-gas system of the engine can be modeled. However, a computation in real time, for example, in a control apparatus of the engine is not possible with such simulation programs because of the high complexity of computation.
It is an object of the invention to improve a method of the type mentioned initially herein so that the desired quantity can be determined with this method with less complexity as to computation and, at the same time, with high precision.
The method of the invention is for operating an internal combustion engine including a combustion chamber, a crankshaft and inlet and outlet valves opening to the combustion chamber. The method includes the steps of: detecting the rpm (nmot) of the crankshaft; considering a fresh air charge (rl) of the combustion chamber and the rpm (nmot) when computing a pressure (ps) in a region lying upstream of the inlet valve by utilizing at least one of thermodynamic equations and flow equations at at least one discrete time point during a work cycle of the engine; or, considering a pressure (ps) in the region and the rpm (nmot) when computing the fresh air charge (rl) of the combustion chamber by utilizing one of thermodynamic equations and flow equations at at least one discrete time point during a work cycle of the engine.
The actual thermal and dynamic conditions in the combustion chamber and in the regions of the engine close to the combustion chamber can be determined with a very high precision with thermodynamic equations and/or flow equations. In contrast to the use of empirical equations and/or of characteristic fields, also the complex thermal and dynamic characteristics of modern internal combustion engines can be simulated very accurately. The computation load of a control apparatus with which functions of the engine are controlled (open loop and/or closed loop) are very low.
The formula or formulas, which result from the thermodynamic equations and/or flow equations for the computation of the fresh air charge and/or of the pressure, need only be computed once during a work cycle of the engine. A continuous small-stepped computation of the instantaneous thermal and dynamic condition in the engine is not necessary in the method of the invention as it is required in conventional simulation programs utilizing mainframe equipment. Furthermore, the influence of the instantaneous temperature of the supplied fresh gas as well as the temperature of the exhaust gas can be simulated physically in a simple manner which likewise contributes to the accuracy of the computed result.
In a first embodiment of the invention, it is suggested that a remainder gas be considered in the computation which is present in the combustion chamber after the closing of the inlet valve. Such a remainder gas is almost always present to a slight extent and is especially present however when the engine has an internal or external exhaust-gas recirculation. In such an internal exhaust-gas recirculation, the opening time point of the inlet valve and/or the closing time point of an outlet valve is so placed that the combustion chamber is filled at the start of a new work cycle not only with fresh air but also with remainder exhaust gas coming from a previous combustion. The flame temperature in the combustion chamber can be reduced by the remainder gas and therefore the formation of nitrous oxide is reduced. The consideration of this remainder gas, which is present in the combustion chamber, is very well possible with the method of the invention.
In a further embodiment, it is suggested that, in the computation, at least one of the following is considered: a residual remaining gas, which is present in the combustion chamber after the closing of the inlet valve and a reaspirated remaining gas, which is present in the combustion chamber after the closing of the inlet valve. In this way, the accuracy in the computation of the fresh air charge or of the pressure in the region lying upstream from the inlet valve is improved still further. With the term xe2x80x9cupstreamxe2x80x9d, that region is meant which is disposed between the inlet valve and the beginning of the intake manifold independently of whether the flow is actually from the intake manifold into the combustion chamber or from the combustion chamber into the intake manifold.
The residual remainder gas is understood to be that remainder gas which is trapped in the combustion chamber volume at combustion chamber temperature and under exhaust-gas counterpressure at the time point of the closing of the outlet valve of the engine. Reaspirative remainder gas is understood to be the remainder gas which flows during the valve overlap (that is, when the inlet and outlet valves are simultaneously open) from a region, which lies downstream of the outlet valve, through the combustion chamber into the region lying upstream from the inlet valve. The reaspirative remainder gas superposes with the residual remainder gas.
The sum of the residual and reaspirative remaining gases defines the total internal remaining gas of the engine. With the subdivision of the remainder gas into a residual component and a reaspirative component, comparatively simple thermodynamic and/or flow equations can be used for computing the respective components. Furthermore, the influences on the various remaining gas components, such as the switching time point of the inlet and outlet valves and the valve overlap, can be still better considered.
The temperature of the gas mixture disposed in the combustion chamber can be determined based on the mixture formula:       T    mix    =                    ∑        i            ⁢                        m          i                *                  T          i                                    ∑        i            ⁢              m        i            
while considering the mass components of the residual remaining gas and/or the mass components of the reaspirative remaining gas and the fresh air and the corresponding temperatures. This formula can be easily computed in the control apparatus and offers good results.
It is especially preferred when, for computing the quantity of the reaspirative remainder gas, which is disposed in the combustion chamber, it is assumed that in specific operating states of the engine, gas from a region, which lies downstream from the outlet valve, can flow through an equivalent throttle into the region which lies upstream of the inlet valve. The quantity of the backflowing gas is computed from an overcritical mass flow, which flows through this throttle. The overcritical mass flow is dependent upon at least an overlapment of the opening angle of the inlet valve with the closing angle of the outlet valve; on a temperature of the gas in the region lying downstream from the outlet valve; on a pressure of the gas in the region lying upstream of the outlet valve; and/or on the ratio of the pressure of the gas in the region, which lies downstream from the outlet valve, to the pressure of the gas in the region lying upstream from the inlet valve.
At this point, it is noted that the term xe2x80x9cdownstreamxe2x80x9d always refers to the region which is disposed between the outlet valve and the end of the exhaust-gas pipe independently of whether the flow is actually from the combustion chamber into the exhaust-gas pipe or from the exhaust-gas pipe into the combustion chamber. The term xe2x80x9cdownstreamxe2x80x9d therefore refers to the direction of the main flow.
The above-mentioned model-like assumption corresponds very well to the actual conditions of the internal combustion engine. The flow of the exhaust gases through the opening of the outlet valve and through the combustion chamber and through the opening of the inlet valve can be very well expressed by a flow of a gas through an equivalent throttle. Such a flow through a throttle can be computed with high precision with the known thermodynamic and aerodynamic equations. The characteristics of the equivalent throttles can be determined in experiments.
The overcritical mass flow can also be dependent upon the position of the centroid of the intersect region of the two valve curves. If the closing speed of the outlet valve is equal to the opening speed of the inlet valve, then the centroid lies precisely below the tip of the approximately triangular intersect region. The centroid shifts when there is a closing speed different from the opening speed and therefore the time point shifts correspondingly at which the above-mentioned values are detected.
It is also advantageous when the overcritical mass flow is multiplied by the output value of a characteristic line into which is fed the ratio of the pressure in a region, which lies downstream of the outlet valve, to the pressure in the combustion chamber or in a region which lies upstream of the inlet valve. Such a characteristic line is identified as xe2x80x9ccharacteristic line outflowxe2x80x9d. This is an equation which is known from flow mechanics and which defines the flow through a diaphragm. With this equation, the flow performance is expressed in a simple manner in dependence upon the pressure difference on both sides of the diaphragm.
Here it is assumed that the gas, which flows back during the valve overlapment, has an exhaust-gas temperature and an exhaust-gas counterpressure. However, when pulsations of the pressure downstream of the outlet valve and upstream of the inlet valve occur in dependence upon the rpm of the crankshaft of the engine, then, during the overlapment, the pressure quotient can assume a value different from its mean value.
In order to consider the above in the computation, it is suggested that the ratio of the pressure of the gas, which lies upstream from the inlet valve, to the pressure of the gas in the region, which lies in the region downstream of the outlet valve, is multiplied by a corrective factor, which is dependent upon the rpm of the crankshaft of the engine.
In another embodiment of the invention, it is suggested that, in the computation, the measured or modeled pressure of the gas in the region, which lies downstream of the outlet valve, is corrected in dependence upon the rpm of the crankshaft of the engine and/or in dependence upon the closing angle of the outlet valve. In this way, the fact is taken into account that the pressure in the region, which lies downstream of the outlet valve, can pulsate in specific rpm ranges and/or when the outlet valve closes at a specific time point within the work cycle of the engine. These pressure pulsations are considered by the correction suggested in accordance with the invention.
In the simplest case, the correction can take place in that the measured or modeled pressure is multiplied by the output of a characteristic field whereinto the rpm of the crankshaft of the engine and the closing angle of the outlet valve are fed. With this correction, it can also be considered that a pressure compensation no longer takes place when the outlet valve closes clearly ahead of top dead center or clearly after top dead center of the piston assigned to the combustion chamber as well as at higher rpms of the crankshaft of the engine. If the outlet valve closes ahead of top dead center of the piston, then the pressure of the residual remaining gas is higher and, in contrast, if the outlet valve closes after top dead center, the pressure of the residual remaining gas in the combustion chamber is lower.
In the same manner, the measured or modeled pressure of the gas in the region, which lies upstream from the inlet valve, can be corrected in the computation in dependence upon the rpm of the crankshaft of the engine and/or in dependence upon the opening angle of the inlet valve.
It is further provided that the mass of the residual remaining gas is determined by means of the combustion chamber volume, which is present at the closing time point of the outlet valve or approximately at the center of the valve overlapment. It would also be possible to use that combustion chamber volume in the computation, which is present when the two valves have the same valve stroke. In all the above cases, the mass of the residual remaining gas can be computed accurately.
For the thermodynamic computations, it is especially preferred to proceed from the status equation for ideal gases. This makes possible significant simplifications in the computation without the result being affected thereby.
According to the invention, it can also be assumed that the thermal capacity and/or the isentropic exponent of the remaining gas or of the components of the remaining gas have the same values as those of fresh air. This assumption is possible because nitrogen is present for the most part in both gases.
A further approximation, which contributes to the simplified execution of the method of the invention, comprises that the state equation for ideal gases is used for adiabatic conditions. In the thermodynamic relationships, thermal transitions at the valves, on the walls of the combustion chamber as well as other components in the combustion chamber and in the regions close to the combustion chamber are neglected. This is possible without deteriorating the accuracy of the computation to any great extent.
The influence of thermal transitions on a detected or modeled temperature can, however, be considered, in the region upstream from the inlet valve by means of a corrective function. In this way, the computation is adiabatic, on the one hand, which makes possible a considerable simplification in the derivation of the equations, while, on the other hand, the influence of thermal transitions is not left completely unconsidered. The computation is thereby possible in a simple and yet precise manner.
The invention relates also to a computer program, which is suitable to carry out the above method when executed on a computer. Here, it is preferred when the computer program is stored on a memory and especially on a flash memory.
The invention also relates to a control apparatus (open loop and/or closed loop) for operating an engine. For such a control apparatus it is suggested that it include a memory on which a computer program of the above kind is stored.